1. Field of the Invention
This invention relates generally to phase tracking networks for correcting the phase characteristics of a signal transmitted through a communication channel and more specifically to the use of such a network in combination with a telephone line providing the signal with frequency offset, phase offset, and phase jitter.
2. Description of the Prior Art
It has been desirable to use telephone lines for communicating data between transmitters and receivers because of their availability and wide distribution. In some cases the telephone lines have been leased to obtain a guaranteed passband. However, the leased lines have been particularly expensive and have been limited in that they have provided a communication channel only between two locations. For these reasons dial-up telephone lines are more desirable although their respective passbands and quality vary considerably.
The telephone lines in general provide a transmitted signal with many undesirable characteristics which are typically corrected at the receiver. For example, the telephone lines provide the transmitted signal with delay and attenuation distortion. The telephone lines have also produced rapid variations in the difference between the phase of the modulation carrier of the transmitter and the phase of the demodulation carrier of the receiver. This variation is generally sinusoidal and is commonly referred to as phase jitter.
As with any transmission channel having delay characteristics, the telephone lines also produce an initial phase offset. The phase offset accounts for some absolute time differential of which the receiver is not initially aware. If the transmitter and receiver are started simultaneously, the clock in the receiver will be ahead of the clock in the transmitter because of the finite time it takes the transmitted signal to reach the receiver. Once the phase offset is corrected, the clocks in the transmitter and receiver will be substantially in step.
Transmission lines have also produced frequency offset wherein the entire spectrum of the transmitted signal is shifted. With respect to time, the frequency offset has generally a linear phase characteristic. Collectively, the phase jitter, phase offset and frequency offset provide the incoming signal at the receiver with an undesirable phase angle.
In some systems, the receiver has included a coherent demodulation followed by lowpass filters defining the desired passband. Following the lowpass filters, an equalization network has been provided to correct the delay and attenuation distortion of the incoming signal. Both the lowpass filters and the equalization network have introduced considerable delays in the system. A phase error signal, which has typically been produced in a circuit following the equalization circuit, has been fed back to the coherent demodulation circuit to drive a voltage control oscillator.
The resulting loop, typically referred to as the phase tracking network, has provided its phase corrections at the coherent demodulation circuit. Therefore, the circuit providing the phase error signal has been responsive to coherently demodulated signals which have also been equalized.
It will be noted that these phase tracking networks typically include the equalization and lowpass filter circuits. Unfortunately, the delays associated with these two circuits have provided the phase tracking networks of the prior art with a low frequency response which has been relatively incapable of tracking fast phase jitter. That is to say, the errors providing an undesirable phase angle in a particular baud interval have not been determined until several baud intervals later, and the resulting corrections have been correspondingly delayed. In some cases this delay has compounded rather than corrected the phase error.